Measurement system with separate optimized beam paths

ABSTRACT

The subject invention relates to a broadband optical metrology system that segregates the broadband radiation into multiple sub-bands to improve overall performance. Each sub-band includes only a fraction of the original bandwidth. The optical path—the light path that connects the illuminator, the sample and the detector—of each sub-band includes a unique sub-band optical system designed to optimize the performance over the spectral range spanned by the sub-band radiation. All of the sub-band optical systems are arranged to provide small-spot illumination at the same measurement position. Optional purging of the individual sub-band optical paths further improves performance.

PRIORITY CLAIM

This application is a continuation of application Ser. No. 10/141,267,filed May 8, 2002, and claims priority to U.S. Provisional PatentApplication Ser. No. 60/329,819, filed Oct. 16, 2001, which areincorporated herein by reference.

TECHNICAL FIELD

The subject invention relates to the field of optical metrology,particularly broadband optical metrology tools that may require acontrolled ambient to eliminate the effects of atmospheric opticalabsorption. Specifically, the invention relates to an optical metrologyinstrument that includes optimized, spectrally segregated opticalsystems and gas purged optical paths to improve system performance.

BACKGROUND OF THE INVENTION

In the prior art there has been considerable effort expended inextending the spectral bandwidth, or wavelength range, of a broad classof optical metrology instruments. Non-contact, optical measurements areheavily utilized in the optics, optical communications and semiconductorindustries. The instruments are used in the evaluation andcharacterization of samples that can include spatially non-uniformdistributions of a broad class of materials including, insulators,semiconductors and metals; consequently, the optical properties of thesesamples can vary markedly with wavelength. Hence, in general, a muchgreater wealth of information can be extracted from broadbandspectroscopic measurements than can be obtained from measurements madeover a narrow spectral range. One approach to implementing broadbandspectroscopic measurements is set forth in U.S. Pat. No. 6,278,519,which is incorporated herein by reference.

At present, leading-edge industrial lithography systems operate in theDUV over a narrow wavelength region of approximately 193 nm.State-of-the art optical metrology systems that operate over thespectral range spanning the DUV-NIR (190 nm-850 nm) characteristicallyemploy two lamps to span this range of measurement wavelengths, aDeuterium lamp for spectroscopic measurements between 190 and 400 nm,and a Xenon lamp for measurements between 400 nm and 800 nm.

In the near future systems will operate in the vacuum ultra-violet at anexposure wavelength of 157 nm. Wavelengths in the range between 140nm-165 nm lie within a region known as the vacuum ultraviolet (VUV), inwhich the high absorption coefficients of oxygen and water vapor lowerthe attenuation length in standard air to fractions of a millimeter.(Historically, this light could only be observed under vacuumconditions, hence the designation.) Achieving the transmission andstability necessary for optical metrology in the VUV, in a tool wherethe optical paths are of order 0.5-2 m requires oxygen and waterconcentrations in the low parts-per-million (ppm) range averaged overthe entire optical path. One approach to achieving this is described incopending U.S. application Ser. No. 10/027385, filed Dec. 21, 2001, nowU.S. Pat. No. 6,813,026, and incorporated herein by reference, whichdiscloses a method for inert gas purging the optical path.

The technology extension requires that the measurement bandwidth ofoptical metrology tools be broadened to cover the wavelength rangespanning 140-850 nm. This is a daunting challenge.

First, optical systems that spectrally segregate the illumination withdiffractive elements must account for and suppress unwanted signalsproduced by harmonic contamination. For example, at wavelength λ adiffraction grating will produce a 1^(st) order an intensity maximum atan angular position θ; at a wavelength λ′=λ/2 the same grating, in the2^(nd) order, also produces an intensity maximum at θ. Consequently,when 140 nm light impinges upon a diffractive grating in addition to the1^(st) order diffracted beam, there are 2^(nd), 3^(rd), 4^(th), 5^(th)and 6^(th) etc. diffractive orders that appear at angular positionscorresponding to 1^(st) order diffraction at 280 nm, 420 nm, 560 nm, 700nm and 840 nm light, etc., respectively. Consequently spectrometers thatuse diffraction gratings to disperse the light must, at a minimum,include order-sorting, optical filters to suppress contributions fromthe higher orders.

Second, most common optical materials undergo solarization, structuraland electronic changes to the material that occur upon exposure to VUVand DUV light. VUV and DUV exposure can significantly modify and degradematerial optical properties. To avoid the adverse effect ofsolarization, refractive VUV optical systems must exclusively employwide bandgap optical materials such as CaF₂, MgF₂, LiF, and LaF3.

Third, virtually all optical materials are dispersive; i.e. the materialrefractive index varies as a function of wavelength, or opticalfrequency. Therefore, the focal position of an individual lens will bewavelength dependent giving rise to chromatic aberration of the lens.This phenomenon complicates the design of broadband optical systems.Chromatic correction can be achieved, to a limited extent, using lenseswith multiple optical elements fabricated from at least two opticalmaterials. The general idea is to select and configure components sothat there is partial cancellation of the chromatic effects, therebyincreasing the useful wavelength range. The designs may be very complexand involve the use of multiple components arranged in multiple groups.The complexity of the optical design and the cost of system fabricationincrease with the optical bandwidth.

Note that, to some extent, requirements two and three above are mutuallyexclusive. It may not be practical to provide a lens design thatsimultaneously exhibits good transmission in the VUV and chromaticcorrection over the 140 to 850 nm wavelength range. As proposed herein,one solution to this problem is to spectrally segregate the broadbandlight and provide multiple lens systems. Each lens operates over alimited wavelength range, or optical sub-band. The sum of the sub-bandsconstitutes the overall system bandwidth. Here, each lens is designed tooperate over a limited wavelength range. This may permit a simpleroptical design to be used in each of the sub-band lenses and provide asystem architecture that provides superior broadband optical performanceat reduced system cost.

Accordingly it would be desirable to provide a metrology toolarchitecture that permits extension of the measurement bandwidth overthe spectral region spanning the VUV-NIR 140 nm-850 nm and avoids theproblems associate with atmospheric absorption, chromatic aberration andharmonic contamination.

The acronyms used in this specification have the following meanings:

CD=critical dimension

DUV=deep ultraviolet

VUV=vacuum ultraviolet

NIR=near infrared

SUMMARY OF THE INVENTION

The subject invention relates to a broadband optical metrology systemthat optimizes performance by dividing the broadband illumination intomultiple sub-bands, such that each sub-band spans only a fraction of thefrequency width of the original broadband spectrum. The sub-bandillumination is transmitted along a unique sub-band optical path thatconnects the illuminator, the sample and the detector. The sub-bandoptical path includes a sub-band optical system that is optimized forthe range of frequencies contained within the sub-band. In this fashion,a single broadband measurement constitutes multiple narrow-bandmeasurements.

Ideally, the individual sub-band optical systems are arranged to producesmall-spot illumination of the sample and are configured such that allsystems illuminate the sample at the same location. In this way, thebroadband information is derived from probing the same spatial region ofthe sample. The highest system throughput is achieved when thesimultaneous measurements are made over all of the sub-bands. Tocircumvent the adverse effects of atmospheric optical absorption(particularly problematic in the VUV) the sub-band optical path can bepurged with an optically transparent gas such as N₂ or He.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 4 are schematic diagrams of a generalized opticalmetrology system comprising illuminator, sample, detector, processorwith two light sources and two separate sub-band optical paths. Fourrepresentative arrangements of the sub-band optical systems areillustrated.

FIG. 1 is a schematic diagram illustrating separate light paths betweenthe source and sample.

FIG. 2 is a schematic diagram illustrating separate light paths betweenthe sample and detector.

FIG. 3 is a schematic diagram illustrating separate paths between thesource and the detector.

FIG. 4 is a schematic diagram illustrating a single broadband source andseparate light paths between the sample and detector.

FIG. 5 is a schematic diagram illustrating common optical paths withseparate detectors

FIG. 6 is a schematic diagram of a preferred embodiment of the inventionincluding a multiple light source ellipsometer with separateillumination sub-band optical paths and detector sub-band optical paths.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 through 6 are schematic and a highly simplified opticalrepresentation has been used to reduce the complexity of the drawings.Herein individual optical components are used to represent functionalelements that may be quite complex; for example, a lens in these figuresrepresents an optical system for collecting and focusing light—thephysical embodiment of the optical system may employ multiple lenses inmultiple groups.

FIG. 1 illustrates an embodiment of the invention 10, wherein twospectrally segregated light sources 12 and 14 are used to illuminate thesample. Light source 12 emits illumination over a first spectrum. Theoptical path 28 traversed by light of the first spectrum includes lightsource 12, optical system 16, sample 20, optical system 22 and detector24. Optical system 16 focuses a portion of the light emitted by source12 onto sample 20 where the light interacts with and reflects from thesample. A portion of the reflected radiation is collected by opticalsystem 22 and focused onto detector 24. Detector 24 generates outputsignals in response to the detected illumination and processor 26records and analyzes those signals.

Light source 14 emits illumination over a second spectrum substantiallydifferent from the first. The optical path 30 traversed by light of thesecond spectrum includes light source 14, optical system 18, sample 20,optical system 22 and detector 24. Optical system 18 focuses a portionof the light emitted by source 14 onto sample 20 where the lightinteracts with and reflects from the sample. A portion of the reflectedradiation is collected by optical system 22 and focused onto detector24. Detector 24 generates output signals in response to the detectedillumination and processor 26 records and analyzes those signals.

Note that in this arrangement the optical paths 28 and 30 in the regionbetween sample 20 and detector 24 are substantially identical. However,those portions of optical paths 28 and 30 between source 12 and sample20, and source 14 and sample 20 are different. The spatial separation ofthe light contained within the first and second spectra permits opticalsystems 16 and 22 to be designed to optimize performance over a limitedspectral range: optical system 16 is optimized for wavelengths includedwithin the first spectrum, optical system 18 is optimized forwavelengths included within the second spectrum.

Optical systems 16 and 18 are configured to illuminate the sample at thesame location with substantially identical spot-sizes. In this way, theaberrations of both optical systems may be reduced which permitssuperior focusing performance and a smaller illuminated region on thesample, which, in turn, permits measurement of smaller regions of thesample and the measurement of smaller features.

The arrangement admits several variations. For example in onearrangement both light sources are broadband, e.g. light source 12 is aXe lamp and light source 14 is a deuterium lamp. In this case opticalsystem 16 is optimized for performance over a wavelength range spanningapproximately 400-850 nm. Optical system 18 is optimized for performanceover a wavelength range spanning approximately 190-400 nm.

Different optimization criteria may be employed in designing opticalsystems 16 and 18. For example, it may be important to insure that anidentical illumination spot is used to probe the sample at allillumination wavelengths, i.e. the optical designs must minimizechromatic aberration. Generally, chromatic aberrations of opticalsystems increase with the range of operating wavelengths; therefore,chromatic correction is more readily accomplished over a limitedwavelength range. Optical system 18 is designed to operate over asub-band of optical frequencies spanning the wavelength range between190 and 400 nm. Similarly, optical system 16 is designed to operate overa sub-band of optical frequencies spanning the wavelength range between400 and 850 nm. Superior correction can be achieved over the entirerange of operating wavelengths since optical systems 16 and 18 may beindividually chromatically corrected over their respective sub-bandfrequencies.

In the present case the optimum design for optical systems 16 and 18 maybe very different, with each system employing a unique set of opticalelements fabricated from unique materials. For example, optical system18 will require the use of higher quality and therefore more expensiveUV compatible optical materials such as UV grade fused silica and CaF₂.Optical system 16 may employ a wider variety of lower cost opticalmaterials.

Alternatively, cost may be the most important factor governing theoptical design and a given degree of chromatic aberration may beacceptable. As discussed above, chromatic correction of two individualsub-band optical systems is simpler and requires fewer optical elementsthan chromatic correction of a single broadband optical system thatspans the entire wavelength range. Since fewer elements are required thecost is reduced.

Additionally, it may be desirable to provide superior environmentalcontrol for one of the sub-band optical systems, e.g. sub-band opticalsystem 18 which operates over the wavelength region spanning 190-400 nm.In this case, and as illustrated in the preferred embodiments of FIGS.3-6, it is desirable to house the optical system in an enclosure purgedwith inert gas. Gas purging can prevent unwanted optical absorption andmitigate against degradation of the sub-band optics due to chemicaladsorption of reactive species on the optical surfaces. The concept ofoptimizing a sub-band optical path can include selection or design oflenses and other optical elements and/or environmental controls whichcan improve the performance of the system.

Lastly, although the preceding discussion has been undertaken withregard to the use of broadband sources, one or both of light sources 12and 14 may be a laser. In this case optical systems 16 and/or 18 areoptimized at the laser wavelength(s).

The arrangement of FIG. 1 may be generalized to include multiple lightsources and multiple optical systems. Furthermore, it may be employed insystems that include one or more optical systems used separately or incombination including reflectometers, ellipsometers, scatterometers, andoptical CD metrology tools operating as single wavelength metrologysystems and broadband spectroscopic instruments.

FIG. 2 illustrates an embodiment of the invention 40, wherein twospectrally segregated light sources 12 and 14 are used to illuminate thesample. Light source 12 emits illumination over a first spectrum andlight source 14 emits illumination over a second spectrum. The opticalpath 28 traversed by light of the first spectrum includes light source12, beam combiner 25, optical system 32, sample 20, optical system 34and detector 24. The optical path 30 traversed by light of the secondspectrum includes light source 14, beam combiner 25, optical system 32,sample 20, optical system 36 and detector 24.

Note that optical paths 28 and 30 are substantially coincident in theregion between beam combiner 25 and sample 20 and that optical system 32is used to provide small-spot illumination of sample 20 with lightoriginating from both of the light sources 12 and 14. Beam combiner 25may be a prism, a beam-splitter, a dichroic mirror, or other suitableelement which functions to combine a portion of the light emitted bysources 12 and 14.

Following reflection from sample 20 light paths 28 and 30 diverge. Aportion of the reflected illumination with wavelengths within the firstspectrum is collected by optical system 34, and a portion of thereflected illumination with wavelengths within the second spectrum iscollected by optical system 36. Optical systems 34 and 36 are separatelyoptimized for the spectral regions spanning the first and second spectrarespectively. In this way, the aberrations of both optical systems maybe reduced which permits superior detection performance.

Optical systems 34 and 36 focus the collected ration onto detector 24.Detector 24 generates output signals in response to the detectedillumination and processor 26 records and analyzes those signals.

The arrangement admits several variations. For example in onearrangement both light sources are broadband, e.g. light source 12 is aXe lamp and light source 14 is a deuterium lamp. Here optical system 34is optimized for performance over a wavelength range spanningapproximately 400-850 nm. Optical system 36 is optimized for performanceover a wavelength range spanning approximately 190-400 nm. In anotherarrangement one or both of the sources 12 and 14 may be a laser. In thiscase optical systems 34 and/or 36 are optimized at the laserwavelength(s).

The arrangement of FIG. 2 may be generalized to include multiple lightsources and multiple optical systems. Furthermore, it may be employed insystems that include one or more optical systems used separately or incombination including reflectometers, ellipsometers, scatterometers, andoptical CD metrology tools operating as single wavelength metrologysystems and broadband spectroscopic instruments.

FIG. 3 illustrates an arrangement of the invention 50 that employsseparate optical paths for each of the spectrally separated lightsources 12 and 14. Light source 12 emits illumination over a firstspectrum. The optical path 28 traversed by light of the first spectrumincludes light source 12, optical system 16, sample 20, optical system34 and detector 24. Optical system 16 focuses a portion of the lightemitted by source 12 onto sample 20 where the light interacts with andreflects from the sample. A portion of the reflected radiation iscollected by optical system 34 and focused onto detector 24. Detector 24generates output signals in response to the detected illumination andprocessor 26 records and analyzes those signals.

Light source 14 emits illumination over a second spectrum substantiallydifferent from the first. The optical path 30 traversed by light of thesecond spectrum includes light source 14, optical system 18, sample 20,optical system 36 and detector 24. Optical system 18 focuses a portionof the light emitted by source 14 onto sample 20 where the lightinteracts with and reflects from the sample. A portion of the reflectedradiation is collected by optical system 22 and focused onto detector24. Detector 24 generates output signals in response to the detectedillumination and processor 26 records and analyzes those signals.

The spatial separation of the light contained within the first andsecond spectra permits optical systems 16 and 22 to be designed tooptimize performance over a limited spectral range: optical system 16 isoptimized for wavelengths included within the first spectrum, opticalsystem 18 is optimized for wavelengths included within the secondspectrum. Optical systems 16 and 18 are configured to illuminate thesample at the same location with substantially identical spot-sizes.

In this way, the aberrations of both optical systems may be reducedwhich permits superior focusing performance and a smaller illuminatedregion on the sample, which, in turn, permits measurement of smallerregions of the sample and the measurement of smaller features. Further,optical systems 34 and 36 are separately optimized for the spectralregions spanning the first and second spectra respectively. In this way,the aberrations of both optical systems may be reduced which permitssuperior detection performance.

The arrangement admits several variations. For example in onearrangement both light sources are broadband, e.g. light source 12 is aXe lamp and light source 14 is a deuterium lamp. Here optical systems 16and 34 are optimized for performance over a wavelength range spanningapproximately 400-850 nm. Optical systems 18 and 36 are optimized forperformance over a wavelength range spanning approximately 190-400 nm.In another arrangement one or both of the sources 12 and 14 may be alaser. In this case optical systems 16 and/or 18 are optimized at thelaser wavelength(s).

The arrangement of FIG. 3 may be generalized to include multiple lightsources and multiple optical systems. Furthermore, it may be employed insystems that include one or more optical systems used separately or incombination including reflectometers, ellipsometers, scatterometers andoptical CD metrology tools operating as single wavelength metrologysystems and broadband spectroscopic instruments.

The separation of the light paths illustrated in FIG. 3 has furtheradvantages. For example, one or more of the measurement systems may bemaintained in a purged environment. In FIG. 3 the metrology systemcomprising source 14, light path 30, optical system 18, sample 20,optical system 36 and a portion of detector 24 is located within apurged region 38. In practice, region 38 may include several isolatedregions and enclosures that surround the system components 14, 18, 36,and portions of 24, and beam transport tubes which surround the opticalpaths that connect the system components and the sample. Region 38 maybe purged of atmospheric constituents using inert gas such as N₂ or Hepermitting improvement of the system performance and extension of theoperating wavelengths into the VUV spectral region, including the 157 nmwavelength (F₂ laser transition) used in leading-edge opticallithography. (See the above cited copending U.S. application Ser. No.10/027385, now U.S. Pat. No. 6,813,026, for discussions of purgingtechnology.)

For VUV applications, optical systems 18 and 36 are optimized forperformance over a spectral range spanning a portion of the spectralregion between 140 and 190 nm and employ optical components made fromVUV compatible materials selected from the group consisting of UV gradefused silica, CaF₂, BaF₂, LaF₃, LiF, SrF₂, MgF₂ and fluorine doped fusedsilica.

The arrangement admits several variations. For example in a preferredembodiment including two broadband light sources light source 12 is a Xelamp and light source 14 is a deuterium lamp that includes a VUV window.A separate, gas purged metrology system is used for measurement over thespectral range spanning the spectral region between 140 and 190 nm. Inthe spectral region between 190 and 850 nm, the remainder of the systemis configured as illustrated in FIG. 1, e.g. both sources illuminate thesample, and a common path optical system 22 and detector 24 areemployed.

The arrangement of FIG. 3 may be further generalized to include multiplelight sources, multiple optical systems and multiple purging systems.Furthermore, it may be employed in systems that include one or moreoptical instruments used separately or in combination includingreflectometers, ellipsometers, scatterometers, and optical CD metrologytools operating as single wavelength metrology systems and broadbandspectroscopic instruments.

FIG. 4 illustrates an arrangement of the invention that employs a singleultra-broadband light source 13. By ultra-broadband we mean to indicatethat the light source emits illumination over a very broad spectralrange including, for example, the DUV and VUV, or the DUV, visible andNIR, or the VUV, DUV, visible, and NIR.

A portion of the illumination 29 emitted by ultra-broadband light source13 is collected by broadband optical system 32 and focused to providesmall-spot illumination of sample 20. The incident illuminationinteracts with and reflects from sample 20. A portion of the reflectedillumination is incident onto beamsplitter 31 that divides the reflectedportion into two spectrally segregated optical beams 28 and 30.

The spectral segregation may be accomplished with a one or more elementsselected from the group consisting of, for example, dichroic mirrors,grating, prisms or their equivalent. Beam 28 comprises a first sub-bandspectrum that includes illumination with wavelengths or frequenciesspanning a first portion of the original ultra-broadband spectrum.Similarly, beam 30 comprises a second sub-band spectrum that includesillumination with wavelengths or frequencies spanning a second portionof the original ultra-broadband spectrum.

In the preferred embodiment illustrated in FIG. 4, beamsplitter 31 is adichroic mirror, which is designed to reflect the first sub-bandspectrum 28 and transmit the second sub-band spectrum 30. Illuminationwithin the first sub-band spectrum reflects from fold-mirror 33 and iscollected and focused by sub-band optical system 34 onto detector 24.Illumination within the second sub-band spectrum is transmitted throughdichroic mirror 31, and collected and focused by sub-band optical system36 onto detector 24. Detector 24 generates output signals in response tothe detected illumination and processor 26 records and analyzes thosesignals.

The spectral segregation of the light contained within the first andsecond spectra permits optical systems 34 and 36 to be designed tooptimize performance over a limited spectral range: optical system 34 isoptimized for wavelengths included within the first spectrum, opticalsystem 36 is optimized for wavelengths included within the secondspectrum. In this way, the aberrations of both optical systems may bereduced which permits superior detection performance.

In this arrangement, the spectrally segregated light reaching detector24 is also spatially separated. Consequently, as illustrated in FIG. 4,detector 24 could consist of two detectors: a first detector optimizedfor the wavelength region comprising the first sub-band spectrum and asecond detector optimized for the wavelength region comprising thesecond sub-band spectrum.

Further, the metrology system comprising source 13, light path 29,optical system 32, sample 20, beamsplitter 31, light paths 28 and 30,fold mirror 33, optical systems 34 and 36 and a detector 24 may belocated within a purged region 38. In practice, region 38 may includeseveral isolated regions and enclosures that surround the systemcomponents 14, 18, 36, and portions of 24, and beam transport tubeswhich surround the optical paths that connect the system components andthe sample. Region 38 may be purged of atmospheric constituents usinginert gas such as N₂ or He permitting improvement of the systemperformance and extension of the operating wavelengths into the VUVspectral region.

The arrangement of FIG. 4 may be generalized to include multiple lightsources and multiple optical systems. Furthermore, it may be employed insystems that include one or more optical systems used separately or incombination including reflectometers, ellipsometers, scatterometers andoptical CD metrology tools operating as single wavelength metrologysystems and broadband spectroscopic instruments.

FIG. 5 illustrates an embodiment of the invention 60, wherein twospectrally segregated light sources 12 and 14 are used to illuminate thesample, and separate detection systems 70 and 72 are employed to detectillumination after reflection from and interaction with sample 20.

Light source 12 emits illumination over a first spectrum and lightsource 14 emits illumination over a second spectrum. The optical path 28traversed by light of the first spectrum includes light source 12, beamcombiner 25, optical system 32, sample 20, optical system 22,beam-splitter 31 and one, or both, of the detectors 70 and 72. Theoptical path 30 traversed by light of the second spectrum includes lightsource 14, beam combiner 25, optical system 32, sample 20, opticalsystem 22, beam-splitter 31 and one or both, of the detectors 70 and 72.

Note that optical paths 28 and 30 are substantially coincident in theregion between beam combiner 25 and beam splitter 31 and that opticalsystem 32 is used to provide small-spot illumination of sample 20 withlight originating from both of the light sources 12 and 14. Beamcombiner 25 may be a prism, a beam-splitter, a dichroic mirror, or othersuitable element which functions to combine a portion of the lightemitted by sources 12 and 14.

Following reflection from sample 20 a portion of the reflectedillumination is collected by optical system 22; the optical system isoptimized over the spectral region spanning both sources 12 and 14.Optical system 22 focuses the collected illumination onto the detectors70 and 72. Beam-splitter 31 divides the focused illumination so thateach detector receives a portion of the focused illumination, andgenerates separate output signals in response thereto.

In FIG. 5, beam-splitter 31 is shown as a spectrally selective element.Illumination originating from light source 12 is directed along opticalpath 28 toward detector 70. Illumination originating from light source14 is directed along optical 30 toward detector 72. This arrangementpermits optimization of the response of each detector over a limitedspectral region, thereby improving detection system performance.

One or both of detector systems 70 and 72 may be located within a N₂purged environment 38 to limit the adverse effects of temperaturevariation, atmospheric optical absorption, etc., and improve thetemporal stability of the detector response. Processor 26 records andanalyzes the detector output signals.

The arrangement admits several variations. For example in onearrangement both light sources are broadband, e.g. light source 12 is aXe lamp and light source 14 is a deuterium lamp. Here detector 72 isoptimized for performance over a wavelength range spanning approximately400-850 nm. Detector 70 is optimized for performance over a wavelengthrange spanning approximately 190-400 nm.

For example, in certain applications it is advantageous to employ arraydetectors. In this case, detector 70 is a UV enhanced photodiode or UVenhanced CCD array. Detector 72 is a photodiode or CCD array selected toprovide high sensitivity and good quantum efficiency throughout thevisible and NIR. In applications requiring narrow band detectors,detectors 70 and 72 are, respectively, photomultiplier tubes withphotocathode materials optimized for the UV and visible-NIR regions ofthe spectrum. In another arrangement one or both of the sources 12 and14 may be a laser. In this case detectors 70 and 72 may be photodiodedetectors optimized at one or more laser wavelength(s).

The arrangement of FIG. 4 may be generalized to include multiple lightsources and multiple optical systems. Furthermore, it may be employed insystems that include one or more optical systems used separately or incombination including reflectometers, ellipsometers, scatterometers, andoptical CD metrology tools operating as single wavelength metrologysystems and broadband spectroscopic instruments.

FIG. 6 illustrates a preferred embodiment of a broadband spectroscopicellipsometry system 80. Ellipsometry system 80 includes two separatemeasurement systems; one optimized for VUV operation, the other foroperation in the UV-NIR spectral range. Detector system 24 may includeseparate VUV 72 and UV-VIS 0 detectors.

The VUV ellipsometer is comprised of a VUV light source 14, opticalsystem 18, sample 20, optics system 36 and detector 72. The VUVellipsometer is isolated from the ambient environment and maintained ina N₂ purged environment 38, in order to remove optically absorbingatmospheric constituent from the optical path 30.

Optical system 18 includes condenser 56, polarizer 58, and focusingsystem 62 and is configured to provide small-spot illumination of thesample with polarized broadband VUV light at a measurement location.Optical system 36 includes collection optics 64, wave-plate 66 andanalyzer 68 configured to collect a portion of the illuminationreflected from the sample and measure the change in the illuminationproduced by interaction with the sample.

Detector 72 generates output signals in response to the detectedillumination at a plurality of wavelengths spanning the spectral regionbetween 140 and 190 nm. Processor 26 records and analyzes those outputsignals. Optical systems 18 and 36 are fabricated from VUV transparentmaterials and components selected from the group consisting of UV gradefused silica, CaF₂, BaF₂, LaF₃, LiF, SrF₂, MgF₂ and fluorine-doped fusedsilica. Ideally, the VUV light source 14 is a deuterium lamp with a VUVwindow. In the preferred embodiment polarizer 58 and analyzer 68 are VUVgrade Rochon prisms and wave-plate 66 is manufactured form MgF₂.

The UV-NIR ellipsometer is comprised of a UV-NIR light source 12,optical system 16, sample 20, optics system 34 and detector 70. TheUV-NIR ellipsometer is illustrated with optical path 28 maintained inthe ambient environment; however, if so desired, this system may also bemaintained in a N₂ purged environment in order to improve theperformance of the UV-NIR measurement system.

Optical system 16 includes condenser 42, polarizer 44, and focusingsystem 46 and is configured to provide small-spot illumination of thesample with polarized broadband UV-NIR light at a measurement location.Optical system 34 includes collection optics 48, wave-plate 52 andanalyzer 54 configured to collect a portion of the illuminationreflected from the sample and measure the change in the illuminationproduced by interaction with the sample.

Detector 70 generates output signals in response to the detectedillumination at a plurality of wavelengths spanning the spectral regionbetween 190 and 850 nm. Processor 26 records and analyzes those outputsignals.

Optical systems 16 and 34 are optimized for the wavelength regionspanning 190-850 nm. In the preferred embodiment, the UV-NIR lightsource 12 includes both deuterium and Xe lamps arranged to providebroadband illumination of the sample over the wavelength range spanning190-850 nm. In the preferred embodiment polarizer 44 and analyzer 54 areRochon prisms.

The VUV and UV-NIR optical systems are arranged so that the measurementregions of the VUV and UV-NIR ellipsometers are substantially identical,e.g. the size and shape of the illuminated area at the measurementlocation and the numerical aperture of the collection systems arevirtually identical in the VUV and UV-NIR.

The arrangement of FIG. 5 may be further generalized to includeadditional light sources and additional measurement systems that employgas purged optical paths. Possible generalizations include systems thatuse one or more measurement technologies employed separately or incombination including instruments commonly known as reflectometers,ellipsometers, scatterometers, and optical CD metrology tools operatingas single wavelength metrology systems and broad-band spectroscopicinstruments.

1. An optical metrology tool for evaluating a sample comprising: a firstlight source emitting a first beam of light; a second light sourceemitting second beam of light including radiation at an ultravioletwavelength; a detector for generating output signals in response toreceived light; a first optical path for directing the first light beamto the sample and for collecting the first light beam reflected from thesample and directing the collected first light beam to the detector,said first optical path being exposed to the ambient environment; and asecond optical path for directing the second light beam to the sampleand for collecting the second light beam reflected from the sample anddirecting the collected second light beam to the detector, said secondoptical path being substantially shielded from the ambient environment.2. A metrology tool as recited in claim 1, wherein said second opticalpath is purged by an inert gas.
 3. A metrology tool as recited in claim2, wherein said second optical path includes beam transport tubes forshielding the second light beam from the ambient environment.
 4. Ametrology tool as recited in claim 1, wherein said second optical pathincludes optical components selected from the group consisting of UVgrade fused silica, CaF₂, BaF₂, LaF₃, LiF, SrF₂, MgF₂ and fluorine dopedfused silica.
 5. A metrology tool as recited in claim 1, wherein saiddetector is a spectrometer.
 6. A metrology tool as recited in claim 1,wherein said first light source is a xenon lamp and said second lightsource is a deuterium lamp.
 7. A metrology tool as recited in claim 1,wherein the second light source emits light at 157 nm.
 8. A metrologytool as recited in claim 1, wherein the second light source is a laser.9. A metrology tool as recited in claim 1, wherein said detectorincludes first and second detector subsystems and wherein said firstoptical path directs the first beam of light to the first detectorsubsystem and wherein said second optical path directs the second beamof light to the second detector subsystem.
 10. A metrology tool asrecited in claim 1, which is configured as a system selected from thegroup consisting of spectroscopic reflectometers and spectroscopicellipsometers.
 11. A metrology tool as recited in claim 1, which isconfigured as a system including both a spectroscopic reflectometer anda spectroscopic ellipsometer.
 12. A metrology tool as recited in claim1, further including a processor receiving output signals from thedetector for evaluating the sample.
 13. A metrology tool as recited inclaim 1, wherein each optical path includes an optical system configuredto produce small-spot illumination on the sample at substantially thesame location on the sample.
 14. An optical metrology tool forevaluating a sample comprising: a xenon light source emitting a firstbeam of light; a deuterium light source emitting second beam of lightincluding radiation at an ultraviolet wavelength; a spectrometer forgenerating output signals in response to received light as a function ofwavelength; a processor receiving output signals from the detector forevaluating the sample; a first optical path for directing the firstlight beam to the sample and for collecting the first light beamreflected from the sample and directing the collected first light beamto the detector, said first optical path being exposed to the ambientenvironment; and a second optical path for directing the second lightbeam to the sample and for collecting the second light beam reflectedfrom the sample and directing the collected second light beam to thedetector, said second optical path being substantially shielded from theambient environment and purged by an inert gas.
 15. A metrology tool asrecited in claim 14, wherein said second optical path includes beamtransport tubes for shielding the second light beam from the ambientenvironment.
 16. A metrology tool as recited in claim 14, wherein saidsecond optical path includes optical components selected from the groupconsisting of UV grade fused silica, CaF₂, BaF₂, LaF₃, LiF, SrF₂, MgF₂and fluorine doped fused silica.
 17. A metrology tool as recited inclaim 14, wherein the deuterium light source emits light at 157 nm. 18.A metrology tool as recited in claim 14, wherein said spectrometerincludes first and second detector subsystems and wherein said firstoptical path directs the first beam of light to the first detectorsubsystem and wherein said second optical path directs the second beamof light to the second detector subsystem.
 19. A metrology tool asrecited in claim 14, which is configured as a system selected from thegroup consisting of spectroscopic reflectometers and spectroscopicellipsometers.
 20. A metrology tool as recited in claim 14, which isconfigured as a system including both a spectroscopic reflectometer anda spectroscopic ellipsometer.
 21. A metrology tool as recited in claim14, wherein each optical path includes an optical system configured toproduce small-spot illumination on the sample at substantially the samelocation on the sample.